GB1602541A - High-frequency electromagnetic resonator - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 602 541

1 ( 21) Application No 19995/78 ( 22) Filed 16 May 1978 + ( 31) Convention Application No 6279/77 ( 19 ( 32) Filed 20 May 1977 in : ( 33) Switzerland (CH) ( 44) Complete Specification published 11 Nov 1981 ( 51) INT CL 3 HOIP 7/00 ( 52) Index at acceptance HIW GX ( 54) HIGH-FREQUENCY ELECTROMAGNETIC RESONATOR ( 71) We, PATELHOLD PATENTVERWERTUNGS & ELEKTROHOLDING AG, a Swiss Company of Glarus, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: 5

The invention relates to a resonator for high-frequency electromagnetic oscillations, having a high quality factor (Q-factor) even with a low volume.

The most important characteristics of a resonator for electromagnetic oscillations are its resonance frequency f and the unloaded quality factor or intrinsic quality factor Q The known forms of construction can be divided, in 10 principle, into open and screened systems The first group includes, inter alia, the Fabyr-Perot resonator, the microstrip resonators and certain dielectric resonators, the second group includes, for example, the various coaxial and cavity resonators, triplate resonators and also dielectric resonators, They are generally used in frequency-determining circuits, for example in the form of waveband filters or 15 band-elimination filters The microstrip and triplate resonators, in particular, are of practical importance in circuits with few high selection requirements as are the coaxial and cavity resonators in circuits with a high to very high selection capacity.

Microstrip and triplate resonators are circuit elements in the asymmetrical and symmetrical stripline techniques respectively Usual forms of construction are the 20 open A/2 resonator, the circular-disc resonator and the circular-ring resonator.

Particular advantages are the satisfactory reproducibility, small structural volume, high operational reliability and inexpensive production A disadvantage, on the other hand, is the low intrinsic quality factor as a result of the high galvanic losses.

In the microstrip resonator, the substantially equally high radiation losses must be 25 added to this, although the dielectric losses scarcely matter Stripline wave-band filters therefore have a relatively high pass-band attenuation and a poor selection capacity They are primarily suitable for circuits where there are no particular requirements regarding the transmission quality.

Dielectric resonators are volume resonators and are used in the form of discs, 30 rings, cylinders, parallelepipeds in striplines and in cavities The group of open resonators can be divided into those which are open in one, two or three dimensions In order that an open resonator may be capable of oscillation, the electromagnetic field must decrease in accordance with an exponential or modified

Hankel function in the open direction(s), which behaviour depends on the 35 dimensions and the material constants of the dielectric body and the particular operating frequency In the resonator which is open in one or two dimensions, the particular quality factor depends on the dielectric and galvanic losses; in the one which is open in three dimensions it depends on the dielectric and radiated losses.

Particularly favourable Q, values are obtained with the resonator which is screened 40 on all sides, if the extent of the screening corresponds at least substantially to double the maximum dimension of the dielectric resonator Higher quality factors than the intrinsic value c tan 6 of the dielectric material (S=loss angle) cannot be achieved, however, with the dielectric resonator.

Although the dielectric resonator is described in detail in the literature, only a 45 few actual examples are found in practice A particular disadvantage is the relatively short spacing of the next higher harmonics Furthermore, the construction of filter structures involves certain problems In order to obtain low pass-band attenuation, very low-loss dielectrics are necessary.

Coaxial A/4 resonators are used, in particular, in the form of coaxialline resonators for multi-selection filters, for example as wave-band filters with comblike or interdigital conductor structures Preferred frequency range: 500 M Hz to about 5 G Hz, and intrinsic quality factors Q at best of 2000-3000 can be achieved 5 Cavity resonators are used predominantly in circuits where a high selection is required with the lowest possible transmission losses, for example for aerial filters in highly sensitive microwave receivers If special cases are ignored, the intrinsic quality factors Q which can be achieved are in the range of 5000-10000.

Disadvantages are the relatively large volume in the lower frequency range and the 10 not inconsiderable mechanical expense In order to save weight, metallised ceramic bodies are also used as resonators, which constructions are likewise expensive.

Experience has shown that in all known kinds of resonator, high intrinsic quality factors can only be achieved by means of large conductor surfaces or large 15 field volumes Closer consideration shows that this fact is effectively a consequence of the isotropy of the particular medium traversed by the electromagnetic field in the resonator space _ -In accordance with the invention, there is provided a resonator for highfrequency electromagnetic oscillations, comprising an electromagnetic shield, a 20 dielectric filament of a material of high permittivity enclosed by the shield, with a space of low permittivity between the dielectric filament and the electromagnetic shield, said permittivities and the length of the dielectric filament being such that, in response to an Eorn wave (circular H field, m= 1,2,3) being excited in the dielectric filament, a standing TEM wave develops in said space 25 In the simplest case, the electromagnetic shield may consist of a circular metal tube and said space is filled predominantly with air The Eo, wave excited in the dielectric filament or wire is preferably the E,1 wave (T Mo 1 mode).

Embodiments of this invention will now be described, by way of examples only, with reference to the accompanying drawings, in which 30 Figure IA comprises a longitudinal section and a cross-section through an embodiment of resonator; Figure 1 B comprises corresponding sections through a filter circuit employing resonators in accordance with the invention, Figure IC comprises the same sections through a second embodiment of filter 35 circuit; Figure 2 shows, in cross-sectional and longitudinal sectional views, the field patterns in the resonator of Figure IA; and Figure 3 shows the relationship between intrinsic quality factor Q and different permittivity values 40 In Figure IA, a preferred construction of resonator according to the invention is illustrated in longitudinal and cross section A dielectric wire I with material constants g, (permeability) and e, (permittivity) and diameter D, is disposed concentrically in a circular cylindrical metal tube 3 having internal diameter D 2.

The medium 2 in the gap-for example air-may have (on the average) material 45 constants,2 ' E 2, and it is a prerequisite that if possible u,2 E 2 <<,u E, The diameter D, and the length 1 of the dielectric wire 1 are selected (see below: Theoretical results) that, at the particular resonance frequency, a standing TEM wave appears at least substantially in the outer space 2.

Figure 2 shows an instantaneous illustration of the course or pattern of the 50 field which develops when the E,1 wave is excited in the dielectric wire according to the invention Because f L 2 C 2 <,tl E 1, the particular field structure is built up in the radial direction from the conductor axis By appropriate selection of the wire diameter D, in comparison with the material constants,u, E, and A 2 E 2 and the particular operating frequency, a field course can therefore always be imposed 55 wherein for E waves, the longitudinal component of the electrical field disappears at the surface of the dielectric wire The electromagnetic field in the dielectric hollow cylinder 2, that is to say in the space between the dielectric wire 1 and the metal tube 3 (see Figure IA) is then precisely equal to that between the inner and outer conductors of a coaxial line (TEM wave) The phase velocity of the 60 electromagnetic waves (=standing wave) propagated in both directions then only depends on the operating frequency and the material constants A 2, ú 2 of the dielectric hollow cylinder 2 With such a selection of the length 1 of the dielectric wire that the particular phase difference at the ends of the wire amounts to I 1,602,541 precisely 1800 (or an integral multiple of this length of wire A/2 or rftultiple of A/2), the resonator according to the invention is obtained.

The interaction (and distribution) of the field components is naturally different in the dielectric wire from that in one which is galvanically conducting The quality factor of the resonator explained must therefore be completely different, as will be 5 shown below, from the case with a conventional coaxial-line resonator, for example.

In the practical case, if possible M,2 =ut=,u O and E 2 =E because then the optimum conditions are present with regard to the influence of these material constants on the quality factor (see below: Theoretical results) In addition, the carrier medium 10 should have as low a loss as possible Appropriate possibilities for holding the dielectric wire I in the metal tube 3 are, for example, supporting through two threearmed webs of a plastics or ceramic material (indicated at 4 in Figure IA) , in the A/2 resonator substantially at the spacing 1/2 (I=effective resonance length), so that the electrical disturbances are mutually cancelled out, in the X resonator 15 substantially at the spacing A/2 (potential nodes), furthermore, fixing of the wire at the end and/or to the wall by means of dielectric pins, filling in the gap with foam material and the like.

With reference to the basic shape of the resonator illustrated in Figure IA, various modifications and further developments are possible A/4 resonators, 20 however, have considerably lower intrinsic quality factors because of the losses in the bottom surface A favourable further development is obtained, for example, by interconnecting two A/2 resonators to form a circular ring circuit The individual forms of construction will be discussed later (see below: Technical progress) in connection with the use in filter circuits, where the explanations of Figures IB and 25 IC are also given.

The resonator is preferably suitable for fixed-frequency operation Fine tuning is also possible within certain limits, however, for example by means of a plunger working capacitively and/or inductively.

With the course of the field according to power functions imposed between 30 dielectric wire and tube wall according to the invention, an excitation of oscillations is not possible without the tube Resonance is not possible without the dielectric wire either, so long as the tube diameter is kept below the limiting diameter Both components are essential for the ability of the resonance system to operate The dielectric wire causes the shaping of the field components so that no 35 longitudinal components appear at the surface of the wire, particularly with the E,1 wave The tube, on the other hand, ensures the existence of the TEM wave in the dielectric hollow cylinder The course of the field in the dielectric wire is only like that in the dielectric resonator in the radial direction, in the longitudinal direction and in the outer space, on the other hand, it is like that of a coaxial A/2 resonator 40 The resonance system does not form either a cavity resonator or a true dielectric resonator, hence the key word for the arrangement: quasi-dielectric resonator, hereinafter also abbreviated as QD resonator.

The favourable behaviour of the resonator only appears above a certain limiting frequency which depends on the tube diameter D 2 selected and the 45 permittivity of the dielectric wire In the limiting case (D,=D 2) which is not of interest here, the arrangement corresponds precisely to a dielectric resonator which is open in one dimension The resonance system can be used up to the frequency range of the mm waves The concrete application is primarily a question of the available dielectrics for the production of the dielectric wire At very high 50 frequencies, materials with relatively low permittivities are sufficient, while in the microwave range down to the dm waves, those with higher to very high permittivity values are necessary.

Theoretical Results The great advantages of the proposed resonator are, in particular, the 55 construction of the quality-factor formula and the behaviour in comparison with known kinds of resonator (stripline, coaxial, cavity resonators) The following expositions apply to strictly circular conductor cross-sections Under certain conditions, however, the results can be transferred to conductors with other crosssectional shapes (see below: Technical progress) for example rectangular, elliptical, 60 or arrangements formed of plates.

a) General relationships The present resonator is a further development of the Electromagnetic I 1,602,541 Waveguide forming the subject of British Patent Application 5272/78 Serial No.

1592622 and also called a "quasi-dielectric waveguide" The relationships referred to therein for a line system with doubly coated dielectric are therefore also decisive here, under the individual parameters Hereinafter they will only be mentioned to the extent that this is necessary to describe the resonator characteristics 5 The particular phase constant /p of the hybrid modes (H Enm waves E Hnm waves) propagated in a line system with stratified dielectric results from the solutions of the so-called intrinsic-value equation of the arrangement in question.

In the present case, these are functions of the material constants of the dielectric wire (,u, E,) and of the dielectric hollow cylinder (p 2, E 2), of the diameter ratio 10 a=R 2/R,=D 2/D, and of the pair of values x, y, according to the relationships x 2 =(w 2)zl E, 32)R 2, y 2 =(tus 2 ú 2-i 32)R 2 (I) in which f=w/27 r signifies the particular operating frequency.

From the equations ( 1), separated according to co and /3, it follows explicitly that 15 X 2-y 2 =w 2 (tz,,-P 2,2)R 2 ( 2) and ( 3) R 1 Pl cl E 1 2 t 2 in which, by hypothesis (dielectric wire in the dielectric hollow cylinder) A 11 > 92 ú 2 must be put 20 For the calculation of the resonator characteristics, the relatively simple special case, according to the assumption that the cooperation of the individual quantities at the particular operating frequency is precisely such that the phase constant has the value =w 12 E 2 ( 4) 25 applies here as with the waveguide previously proposed /3 then only depends on the angular frequency 6 o and the material constants p 2, ú 2 of the dielectric hollow cylinder If, in particular, p 2 =io, ú 2 =E, then the propagation velocity of the electromagnetic waves corresponds precisely to the velocity of light in free space.

The introduction of equation ( 4) into ( 1) again leads to y= O and therefore, in 30 accordance with the eigen-value equation to J.(x)= 0 or x=u nm, ( 5) for H Em waves (unm=mth root of the Bessel function of the nth order) In the special case n= 0 Jo(x)= O or x=uom (= 2 4048 for m=l)( 6) for Eom waves With the above pair of values x, y=unm, 0, the particular wire diameter D, necessary for the 35 particular H Enm waves of interest can be given immediately from the equations ( 3) and ( 4) After slight transformation it follows for this that Unm X l 11 =: arl Erl Ir 2 Er 2 ( 7) in which A signifies the operating wavelength in free space and /l,, Cr now signify the relative material constants 40 With regard to attenuation and quality factor, as explained for the waveguide previously proposed, only the case n= O for the H Enm waves (preferably the E,, 1,602,541 1,602,541 5 wave) provides reasonable values The relationships for the E Hnm waves including the Hom waves are therefore omitted here.

b) Resonance frequency The case which is of most interest in practice, namely an open A/2 resonator (according to Figure IA) excited to the Eo 1 wave (m=l), will be assumed With a 5 given resonance wavelength Ao in free space and given material constants, it therefore follows from equation ( 7) with unm=uoi= 2 40482 for the particular wire diameter that il 1 Uf 10 X ( 8) | J Irrlúr r 2 úr 2 and from equation ( 4) with /Il=n for the necessary length of wire 10 1 " ( 9) 2 2 r 2 úr 2 In equation ( 9) any edge effects are omitted for the sake of simplicity The error should, however, amount at most 10 %, as in the conventional,/2 resonator Thus the relationship of the diameter of the resonance element to its length is I r 2 úr 2 ( O 201 r 2 r 2 ( 10) 15 T r Irl Er 1 Ir 2 %r 2 ( 2 uo,/-= 1 531) In the case which is of particular interest in practice r,2 =/r=l, Er 2 l 1 and Er,=Er therefore 0 Uo 1 A O 1 Xo ( 11) TE j Er_l and O 1 = ( 12) 20 2 Thus two conditions have to be adhered to for the dimensioning of the resonance element Equation ( 11) provides such a wire diameter value that a standing TEM wave appears in the space outside the dielectric wire, and equation ( 12) gives the corresponding resonance length.

In addition, care must be taken to ensure that the housing surrounding the 25 resonator does not produce any disturbing resonances This case can be excluded from the beginning if the tube diameter D 2 is selected at most so large that it is always below the limiting diameter for the E 1 wave (without resonance element).

Thus the equation of condition uo 1 Ao D 2 < E o ( 13) 30 D 2 "n:j I'r 2 úr 2 applies, or together with equation ( 9) because 2 uo O =l 531 7 r D 2 < 1 51 ( 14) which requirement can practically always be adhered to.

The screening tube can then also be left open at the ends without energy being radiated.

c) Intrinsic quality factor In the case y= 0 the field components only follow Bessel functions in the dielectric wire, outside the wire they are pure power functions With the H Enm, 5 waves, moreoever, there are no longer any longitudinal components present in the space outside the wire Consequently the energy stored in the resonator and the galvanic and dielectric losses and hence the intrinsic quality factor can be calculated explicitly precisely On the assumption that the substance between dielectric wire and metal tube is free of loss (it being assumed that the field 10 distribution in the resonator suffering from loss is roughly the same as in the case without loss) and ignoring the end losses, the general formula Pl + + tanh 2 (n In a)+ N tanh (n -in a) 1 El 2 an (n -n % tan 6 + o Pa 2 + t E 2 Rn R 2 cosh 2 (nin a) ( 15) is obtained for the H Enm waves in which S designates the loss angle of the dielectric wire, /u L the permeability of the screening tube and 15 1 / ?o cm ( 16) 27/ 300 r)lr L the extent of penetration of the electromagnetic field into the tube wall (o=electrical conductivity in S/cm) Equation ( 15) is written as the individual terms result directly from calculation so that the influence of the various parameters on the quality factor can be recognised immediately 20 In the case which is of particular interest in practice, namely for Prc= 1 Ur 2 = 1 r 1 = 1 and E,2 =l, Er,=Er, it follows from equation ( 15) that 1 + cr tanh 2 (n N a)+ N tanh (n N a) Y E+ Fr tanh 2 (n -In a)l -tan 6 +, o 1) R 2 cosh 2 (n-En a) ( 17) (valid for H Enm waves, n= 0,1,2,3), and according to equation ( 7) with a given tube diameter D 2 now 25 Unm= __ ^ O J 1 ( 18) signifies the particular diameter ratio a=D 2/D 1 It should be noted that a must always be > 1 Er must therefore have a specific minimum value for each unm root.

The condition for t follows from equation ( 18) for a=l as Er > 1 + ( 19) 30 1,602,541 7 1,602,541 7 Equation ( 17) now shows a very interesting behaviour or n>>rlit follows first that Of -cot 6 ( 20) The intrinsic quality factor corresponds to the material quality factor of the dielectric wire and does so largely independently of the quantities Er, N and a On S the other hand if n= 0 (dominant mode) then it follows from ( 17) that 0 = 00 = 1 + 2 1 N a y=o tan + -to ( 21) R 2 In this case the intrinsic quality factor increases continuously as Er increases and does so in proportion to In a, a being given by equation ( 18) for n= 0.

Theoretically, therefore, with very high Er values, the intrinsic value of infinity can 10 be achieved, and this is so regardless of the galvanic and dielectric losses The reason for this behaviour, as the calculation shows, lies in the fact that the energy for n>l is stored predominantly in the dielectric wire, whereas for n= O it is mainly stored outside the dielectric wire The field components and hence the energy density can, in this case (for n= 0) assume very high values at the outside of the 15 surface of the wire as the diameter of the wire decreases, so that the energy storage is predominantly effected only there This also explains the fact that as the ratio a=D 2/D, increases, the influence of the galvanic and dielectric losses is reduced to the same extent.

In Figure 3, with reference to an example, the behaviour of the intrinsic value 20 is illustrated, calculated according to equation ( 17), as a function of the permittivity Er for n=O, 1, 2, 4, 8 and m= 1 Assumptions: f = 10 G Hz and A O = 3 cm, internal diameter of the screening tube D 2 = 10 mm, furthermore tan 8 = 2 10-4, a60 101 S/cm Whereas for n>l the intrinsic quality factor very soon tends towards the quality factor ctg 8 = 5000 of the dielectric wire, for n= O it increases constantly 25 Even with relatively low er values, there are considerable differences For Er= 100, for example, Q already= 12000, and here a= 4 33, that is to say the diameter of the dielectric wire is still 2 31 mm with a length (according to equation ( 12)) of 1 = 15 mm.

Of all possible modes, the E,,, waves are the only ones with which the intrinsic 30 quality factor increases constantly with increasing permittivity of the dielectric wire The most favourable case is for m= 1 (first root of Jj(x)= 0, x=u 01 = 2 4048) because then, according to equation ( 7), the necessary wire diameter D, assumes the smallest value or according to equation ( 18) the ratio a=D 2/D 1 has the highest value with given quantities D 2/A,, and Er 35 As equation ( 15) shows for n= 0, after insertion of the quantities v and a from the equations ( 16) and ( 18), Q becomes the greater, the greater Er, D 2 and f O are.

The alteration of Q is monotonic and unidirectional throughout Here there are no extreme optimisation conditions as is the case, for example with the attenuation constant of the corresponding waveguide 40 As equation ( 15) shows with regard to the influence of the other material constants for n= 0, the intrinsic quality could be additionally increased by making r 2 > 1, that is to say filling in the space between dielectric wire and screening tube, for example with a ferrite Such permeable substances also have a relative permittivity> I, however, and in addition they also suffer from a loss angle so that 45 the intrinsic quality factor would tend to be lower rather than higher as a result.

The case pr>I as also a waveguide of a permeable substance (Ur L>l) would likewise lead to a lower intrinsic quality factor Furthermore, the ratio E 1/2, only contained in a=D 2/D 1 for n= 0 in equation ( 15) (see equation ( 18)), should be as great as possible The above assumption l Pr L=Pr 2 =flr 1 =l and Er 2 = 1 therefore provides 50 the optimum conditions with regard to the influence of these material constants on the intrinsic quality of the resonator.

Assuming a loss angle 82 in the dielectric hollow cylinder, the denominator in equation ( 21) assumes the form N=tan ( 8,)+ 2 tan ( 52)In a+ ( 22) 55 R 2 8,0,4 The favourable behaviour of Q according to equation ( 21) is then no longer present As E,, increases, Q tends to ctan 52, so that the losses in the dielectric hollow cylinder should be kept as low as possible.

In the case of the A/4 long QD resonator ( 1 =( 2 p-1) AJ 4, p= 1,2,3) the denominator in equation ( 21) is replaced by the expression 5 N=tan ( 8 j+ + ( 1 + 2 In a) ( 23) R, 1 Here, too, the favourable behaviour of Q according to equation ( 21) is disturbed As e, increases, Q tends towards I/v O The galvanic losses in the screen are less important, however, the greater the length 1 of the resonance element can be made The same also applies to the A/2 QD resonator short-circuited at the ends 10 If a resonator only contains pure line losses, its intrinsic quality factor is independent of its length Any end losses are always distributed in their effect over the whole length of the resonator They therefore play a less important part, the longer the resonator is.

Even in the proposed QD resonator which is open at the ends, appreciable end 15 losses can occur with certain diameter ratios, as a result of field distortion These can easily be reduced to an insignificant extent, however, by appropriate dishing of the end faces of the dielectric filament or rounding off of the end edges of the dielectric filament They disappear entirely if the QD resonator consists of a circular ring circuit, for example 20 d) Comparison with coaxial line resonators The excellent behaviour of the QD resonator can be seen from the example shown in Figure 3 The advantages, for example in comparison with the stripline resonator, namely considerably higher intrinsic quality factor with substantially the same dimensions, are obvious Considerably better quality factors can also be 25 achieved in comparison with the dielectric resonator In principle, even the very high quality factors of the cavity resonators can be achieved, although materials with comparatively high ú, values are necessary for this The exceptional characteristics of the QD resonator can be seen, in particular, from a comparison with the behaviour of the conventional coaxial-line resonator 30 Assuming the same material constants of the conductors and air as an intermediate medium, the intrinsic quality factor of the open A/2 coaxial resonator is determined by Inb D QKA ( 24) l+b D in which the diameter ratio b=D/d is present not only in the numerator but also in 35 the denominator The maximum of this function is b,e=b O _ 3 6 Thus for the maximum possible value QKA Q KA= D ( 25) The quantities b and D are independent of the particular resonance frequency.

The comparison of equation ( 25) with ( 21) now provides an equation of condition 40 for what loss angle the material of the dielectric wire should have at the most in order that the intrinsic quality of the QD resonator may be equal to or higher than that of the conventional A/2 coaxial resonator For A=Xo and D=D 2 it follows with equation ( 25) after some transposing that tan6 (< L 3 t 2 In () KA( 2)5 O l 26) / 45 in which a=D 21 DD with given quantities D 2/O and E, is determined by equation ( 18) (unm=uo O = 2 40482) The particular maximum permissble value increases in I 1,602,541 R proportion with In a For example for a=b and Q O KA= 2500, the requirement follows: tan S< 12 10-4 Only a very poor loss angle of the dielectric wire could appreciably adversely affect the competitiveness of the QD resonator with the conventional coaxial resonator as regards quality.

Functionally, the QD resonator behaves like a conventional coaxial-line 5 resonator, the internal conductor of which is an infinitely good conductor and the outer conductor of which has a correspondingly lower conductivity An open; 1/2 coaxial resonator, wherein the conductivity of the inner conductor is assumed to be u,=oo, with v according to equation ( 16), has the intrinsic quality factor a KA = 2 TO tr b ( 27) 10 in which b=D/d can now be desired and ur signifies a correspondingly modified conductivity of the outer conductor With A=A, D=D 2 and v from equation ( 16), the comparison with equation ( 21) with regard to numerator and denominator gives the identities is +In a=ln b ( 28) 15 21 D Xo 1 tai 6 + D ( 29) u 02 3 Oo 1 =ta 5 2 TED 2 r Ft O 2O a and hence the associated diameter of the inner conductor as D, d=( 30) (e= 2 71828) and for the resulting conductivity Le+ T O D 2 3 cr tanrj 2 ( 31) 20 The denominator in equation ( 31) is independent of the ratio a=D 2/D, The losses of the dielectric wire actually appear in the form of additional losses in the outer conductor This transformation effectively means that, according to equation ( 21), the intrinsic quality factor is influenced only in the numerator depending on In a, (in contrast to the conventional coaxial-line resonator, see equation ( 24)) and can 25 therefore assume any high values for very small diameters of wire (a-foo) Formally, the QD resonator corresponds precisely to a conventional coaxial-line resonator which is open at the ends and the inner conductor of which has an infinitely high conductivity, that is to say is to some extent superconducting.

Technical Progress 30 Whereas all known kinds of resonator require a large structural volume for a high quality factor with minimum dielectric losses (tan 8 = 0), a high intrinsic quality factor can be achieved with the proposed resonator, even with a small volume.

Through the dielectric wire, the energy density is concentrated to an increasing extent on the vicinity of the surface of the wire with increasing permittivity but the 35 wire itself is increasingly decoupled from the surrounding field In the limiting case of a very high permittivity, the energy storage is effected practically only in the centre of the screening tube along the surface of the filament-shaped dielectric conductor As explained in the previous paragraph, extremely high quality factors can be achieved It is a prerequisite for this phenomenon that only an electrical 40 1,602,541 radial field should be present at the surface of the wire This is weaker by the factor

E,1/E in the dielectric wire than outside the wire and accordingly also the proportion of energy stored in the wire With the selection of the wire diameter so that in the fundamental mode (E,1 wave), a standing TEM wave appears in the space between wire and screening tube at the particular resonance frequency, this condition is 5 necessarily fulfilled With all other field structures of the H Enrn waves (n= 1,2,3) and E Hnm waves (n= 0,1,2,3) an Bp component is always present In accordance with the transfer conditions for tangential fields at boundary surfaces, however, this is equally great inside the wire as outside the wire The proportions of energy stored in the wire with these modes are also correspondingly high as are the associated 10 losses, so that here at most an intrinsic quality factor corresponding to the quality factor ctan 8 of the dielectric wire can be achieved The E,,, waves (particularly the E,1 wave) are in fact the only types with which such a high intrinsic quality factor can be achieved with the smallest volume.

With the dielectric wire dimensioned on the fundamental mode (E,,, wave), 15 only this wave can exist in the immediate vicinity Higher modes are only possible with correspondingly higher frequencies, in the A/2 QD resonator explained, for example with regard to radial direction according to U,u 02 = 5 5201 with f= 2 3 f, with regard to longitudinal direction with f= 2 f'0 The actual value should lie in between Here, therefore, the next higher natural resonance is at least above double the 20 fundamental frequency, which distance is considerably more favourable in comparison with that in the dielectric resonator.

The proposed resonator is of fundamental importance For the first time, a resonance system for electromagnetic oscillations is disclosed which includes the limiting case (for that is to say D,-40, D, 0, but as small as desired) of an 25 infinitely high intrinsic quality factor with a disappearing volume of the energy storage, independently of the particular galvanic and dielectric losses This characteristic is possible because the QD resonator, as explained in the previous paragraph, formally corresponds precisely to a coaxial line resonator, the inner conductor of which has an infinitely high conductivity In practice, it should be 30 possible to approach this ideal case to the extent desired provided the dielectrics necessary for this are available In the higher frequency range, appreciably high quality factors can be achieved already with comparatively low E, values, while in the microwave range down to the dm waves, higher to very high permittivities are necessary 35 As explained with reference to the circular coaxial resonator form, the dimensions of the dielectric wire are selected so that a standing TEM wave develops at least substantially in the space between wire and screen wall with a given permittivity and resonance frequency As mentioned, these field components are pure power functions, and therefore obey the two-dimensional potential 40 equation and hence also the calculating rules of the conformal representation It can be concluded from this that the results explained here for the coaxial resonator also apply, at least approximately, to, conductor forms which can be derived from the field between two concentric circles by conformal representation These include, for example, rectangular and elliptical cross-sectional shapes, a dielectric 45 wire between metal plates and the like For each such cross-sectional shape of the Q D resonator, with analogous excitation of the E,, wave, there must always be conditions with regard to dimensions and resonance frequency, wherein the electrical lines of force are everywhere perpendicular to the surface of the wire.

Otherwise there would be contradictions in the course of the field during the back 50 transformation of the conductor contours to the circular shape.

In principle, the QD resonator can be realised in all those structural forms which are known from the art of the conventional coaxial-line resonator and its modifications The advantageous behaviour of the proposed resonator only fully applies if the space between dielectric wire and screening tube has as low a loss as 55 possible, with as large an E,1/E 2 ratio as possible, and no radiation and end losses are present The stretched A/2 QD) resonator which is open at the ends and coaxially screened can be regarded as a basic form Its application lies predominantly in the microwave range Possible further developments are, for example, the circular ring circuit, consisting of 2,4,6 A 12 resonators connected in series and a dielectric wire 60 of A/2 total length taken spirally along the screen wall or between two coaxial cylinders (with suitable spacing) Annular QD resonators are suitable into the frequency range of the mm waves, while spiral structures may be considered primarily in the range of the dmn waves.

In principle, the dielectric wire may consist of any antimagnetic material, for 65 1,602,541 example of plastics, ceramic, glass or a liquid embedded in an insulating tube In the present case, ceramics and glass are of particular interest because of the necessary mechanical stability Various suitable ceramic materials have a permittivity between ú,= 10-100 with a loss angle of tan &=( 0 7-5) 10-4 There also exist certain mixed ceramics which contain titanium, as well as those which contain 5 zirconium, strontium and barium, some of which have very high E, values, but also relatively high loss angles Low-loss glasses are known, for example from the art of optical glass fibres It is a condition of their suitability, however, that the static permittivity should be considerably higher than that at light frequencies The concrete use of a specific material depends primarily on its electrical 10 characteristics and the particular operating frequency In the range of very high frequencies, where the dielectric wire has comparatively small dimensions, relatively expensive materials (for example monocrystalline) might possibly be considered for this.

In relation to a given intrinsic quality factor, the loss angle of the dielectric 15 wire may be the greater, the higher its permittivity is With very high permittivity values, therefore, materials with a relatively poor loss angle may also be used.

A preferred field of application for the QD resonator is filter circuits, particularly in the frequency range of microwaves up to the range of the mm waves, for example in the form of band-pass filters, band-elimination filters and the like 20 The individual resonators can easily be assembled in block or plate form The coupling can be effected by the usual methods, such as hole coupling working capacitively or inductively, line coupling etc With thin dielectric wires (for example with D 1 l mm), a filter construction in accordance with the stripline technique is advantageous, preferably in triplate form (no radiation losses) The 25 structural forms known in this filter technique, such as A/2 "endcoupled" and A/2 "side-coupled" filters can also be used here, in principle Suitable carrier media are, for example, plastics, ceramic or glass-like foam materials If particularly lowloss dielectrics are used, considerably intrinsic quality factors are obtained even with relatively narrow plate spacing Thus the quasidielectric resonator opens the 30 way to be able to build highly selective and low-attenuation filter structures in the stripline technique, such as could hitherto only be realised by means of the voluminous cavity resonators.

Two examples of the interconnection of QD resonators to form threecircuits filters are illustrated diagrammatically in Figures l B and IC In Figure IB, the 35 resonance elements I are accommodated in three separate compartments 5 situated side by side and are magnetically coupled through holes 6 in the partition walls Dielectric hollow cylinders 7 serve to support the resonance elements The coupling of the filter to the supply lines is effected capacitively by means of the pins 8 Figure IC shows an arrangement of the three QD resonators in the sense of A/2 40 side-coupled" filter in triplate technique Present between the conductor plates 9 with the carrier substrates 1 o is a third insulating layer 11, the thickness of which is equal to the diameter of the resonance elements I and which contains recesses 12 to receive the resonance elements at points which are selected so that precisely the necessary filter characteristic appears with the 45 particular coupling factors The filter is adapted to the characteristic impedance Z O of the circuit by means of the supply lines 13.

Concrete possibilities for realising the proposed QD resonator already exist because various suitable dielectrics are already known The general use of the resonator, particularly for filter circuits in stripline form, is predominantly a 50 technological problem In many cases in the transmission art, particularly where it is a question of highly selective and/or low attenuation filter structures with minimum dimensions, the resonator could advantageously replace the present arrangements (stripline filter, coaxial and cavity resonators).

Claims (19)

WHAT WE CLAIM IS: 55

1 A resonator for high-frequency electromagnetic oscillations, comprising an electromagnetic shield, a dielectric filament of a material of high permittivity enclosed by the shield, with a space of low permittivity between the dielectric filament and the electromagnetic shield, said permittivities and the length of the dielectric filament being such that in response to an Eom wave (circular H field, 60 m=l,2,3) being excited in the dielectric filament, a standing TEM wave develops in said space.

2 A resonator as claimed in Claim I in which the permeability of said space and the permeability of the dielectric filament are equal to the permeability of I 1,602,541 1 1 1 1 vacuum and in which the permittivity of said space is equal to or greater than the permittivity of vacuum, while the permittivity of the dielectric filament is considerably greater.

3 A resonator as claimed in Claims I or 2 in which said space is filled predominantly with air 5

4 A resonator as claimed in any preceding Claim in which the electromagnetic shield comprises a circular cylindrical metal tube.

A resonator as claimed in any one of Claims I to 3, in which the electromagnetic shield comprises, at least one metal plate.

6 A resonator as claimed in any preceding Claim, in which the dielectric 10 filament comprises a plastics material.

7 A resonator as claimed in any one of Claims I to 5, in which the dielectric filament comprises ceramic material.

8 A resonator as claimed in any one of Claims I to 5 in which the dielectric filament comprises a glass wire 15

9 A resonator as claimed in any one of Claims I to 5 in which the dielectric filament comprises a mono-crystal.

A resonator as claimed in any preceding Claim in which the dielectric filament has a cross-section which is at least substantially circular.

11 A resonator as claimed in Claim 4 or any one of Claims 5 to 10 appended to 20 Claim 4, in which the dielectric filament is disposed concentrically in the interior of the shield.

12 A resonator as claimed in any preceding claim, in which the end faces of the dielectric filament are dished and/or the end edges of the dielectric filament are rounded 25

13 A resonator as claimed in any preceding Claim in which the shield is tubular and the dielectric filament extends helically therealong.

14 A resonator as claimed in any one of Claims 1 to 12, in which the dielectric filament extends spirally between two metal plates.

15 A resonator as claimed in any one of Claims 1 to 11, in which an even 30 number of A/2 resonators are interconnected to form a circular ring circuit.

16 A resonator as claimed in Claim 1 and substantially as herein described with reference to the accompanying drawings.

17 A resonator as claimed in any preceding claim, in combination with means for exciting an E,, wave (circular H field) in the dielectric filament 35

18 A resonator as claimed in Claim 17, in which said E rn wave is an E,1 wave (TMO 1 mode).

19 A filter circuit comprising a plurality of resonators each as claimed in any preceding Claim, particularly for the frequency range of the microwaves to the millimetric waves, in which the individual resonators are assembled in block or 40 plate form and coupling is effected by means of hole coupling, working capacitively and/or inductively, or line coupling.

A A THORNTON & CO, Chartered Patent Agents, Northumberland House, 303/306 High Holborn, London, WCIV 7 LE.

Printed for Her Majesty's Stationery Office, by the Courier Press, Leamington Spa, 1981 Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A IAY, from which copies may be obtained.

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